Exhaust gas from an internal combustion engine such as a diesel engine contains particulates such as soot. In order to control or restrain the amount of particulates contained in exhaust gas which is exhausted from an internal combustion engine to the atmosphere, a particulate sensor for detecting the amount of particulates in exhaust gas may be attached to an exhaust gas pipe of the internal combustion engine. Such a particulate sensor is known to detect the amount of particulates in exhaust gas by generating ions through corona discharge, and electrically charging particles in exhaust gas by use of portion of the generated ions (Patent Document 1, etc., mentioned below).
The particulate sensor mentioned above has a floating potential section whose electric potential varies with the amount of ions used for electrically charging particulates, and a chassis ground section having the same chassis ground potential as that of piping such as a metal exhaust gas pipe to which the particulate sensor is attached. The particulate sensor detects current which flows between the floating potential section and the chassis ground section, thereby detecting the amount of particulates in exhaust gas. The floating potential section and the chassis ground potential section are electrically insulated with an insulating member at portions other than those used for detecting the current.
However, in a conventional particulate sensor, main components are composed of electrically conductive members, and these components vary in electric potential with the amount of ions used for electric charging. That is, the major components constitute a floating potential section. Furthermore, such electrically conductive members are partially exposed to the interior of the exhaust gas pipe. Thus, in the case of adhesion of particulates, water, oil, etc., contained in exhaust gas to that portion of the floating potential section which is exposed to the interior of the exhaust gas pipe, and to the exhaust gas pipe formed of an electrically conductive member, a short circuit is established therebetween. In such a condition, the electric potential of the floating potential section fails to accurately reflect the amount of ions used for electric charging. As a result, the particulate sensor fails to accurately detect the amount of particulates in exhaust gas.
Such a problem is widely involved in not only particulate sensors used for detecting soot in exhaust gas of internal combustion engines but also particulate sensors for detecting the amount of particulates in gas which contains particulates, through utilization of a configuration in which electric potential varies with the amount of ions used for electrically charging particulates or the amount of ions not used for electrically charging particulates.